In addition to the information presented to you on the device's LCD screen when you enable diagnostics mode the Garmin will also be transmitting test data via the RS-232 port. Plug the GPS device into your serial port via the cable included with the unit (or on the Mac via a USB converter) and then use your favourite terminal application to output the data (on Mac OS X the 'cat' command is perfect).
On my eTrex Legend the data consists of a chunk of data transmitted every second formated in the proprietary Garmin GPS protocol. The chunk of data is actually the machines date and time. Just like the diagnostic info provided via the screen this information of limited use to the average user - but anyone writing software to communicate with the Garmin might have a use for it. I found it useful whilst writing my Mac OS X GPS software (Geode), it was useful for testing the Garmin protocol's lame checksum algorithm.